Before the age of compact cars, laptop computers and pocket telephones, there were miniature nuclear warheads.

For as long as there have been engineers, they have been working on making complicated things smaller and better. Weapons are no exception.

Now, North Korea apparently has figured out how to make a very big explosive small enough to sit atop one of its mobile-launched missiles, a development that could threaten much of the U.S., according to a U.S. intelligence report that surfaced this week.

North Korea is making progress, showing it can put together competent teams of scientists and solve technical problems, but it is far from proving that it is capable of launching a punishing nuclear strike on the U.S., according to U.S. weapons experts.

Making a miniature nuclear weapon that has a large explosive force involves a lot of scientific and engineering know-how.

The “Little Boy” bomb that the U.S. dropped on Hiroshima on Aug. 6, 1945, weighed as much as two 2017 Cadillac Escalade SUVs, about 9,700 pounds. Three days later, the “Fat Man” bomb, slightly heavier at 10,300 pounds, was dropped on Nagasaki.

Since then, the weight of U.S. atomic bombs has shrunk considerably, as scientists have refined the physics of the devices and streamlined how they are armed.

With the last generation of nuclear weapons designed in the 1980s, engineers at Los Alamos National Laboratory produced the W88, weighing only 800 pounds despite having an explosive force equal to 475,000 tons of TNT — in other words, less than one-tenth the weight of the first atomic bomb, but 400 times more powerful.

What technical capability is necessary to build a missile-ready nuclear bomb?

The first step is understanding how to reduce the amount of conventional high explosives that surround a hollow pit of highly enriched uranium or plutonium. A nuclear detonation occurs when the high explosive implodes the hollow sphere of fissile material next to it to start an uncontrolled chain reaction.

After the war, work progressed on smaller bombs. One of the crucial design steps was to create a small, precisely uniform air gap between the conventional explosives and the sphere of nuclear fuel, amplifying the force of the conventional explosion and reducing the amount needed to trigger a nuclear chain reaction.

It’s unclear that Pyongyang has mastered that precise construction, said Jeffrey Lewis, a nuclear weapons analyst with the James Martin Center for Nonproliferation Studies in Monterey, Calif.

What Pyongyang has said so far is that its weapon is a “Korean-style mixed charge” device, indicating “they don’t have a lot of plutonium, so they are mixing it with uranium,” Lewis said.

It is possible the North Koreans are also injecting tritium gas into the hollow sphere to get some fusion energy out of the bomb, as well, he said.

“The concept is well known, but you can’t know without testing. But North Korea tests, so they would know,” he said.

North Korea has probably not succeeded in building a lightweight, miniaturized bomb, as the U.S. and Russia have, but only a more compact weapon that isn’t significantly lighter.

The biggest stride in miniaturization involved the hydrogen bomb design pioneered by two Eastern European immigrants, Edward Teller and Stanislaw Ulam.

The Teller-Ulam configuration created two or three stages in a weapon, in which a fission trigger causes X-rays to compress a secondary stage of the weapon containing fusion fuel. The secondary stage can trigger a third stage that contains more fission fuel.

The first full-scale demonstration of such a thermonuclear weapon was conducted in 1952, just seven years after the first atomic bomb test of the Manhattan Project. It created an unexpectedly large blast equal to 10 megatons of TNT.

The “bomb” was actually a machine that weighed an astounding 82 tons, including cryogenic cooling equipment.

The ensuing decades led to refinements that drastically reduced its weight. By the time of the Reagan administration, the U.S. was able to field Peacekeeper MX missiles that could carry 10 warheads each and drop them on separate targets anywhere in the world.

North Korea is believed to have been working on nuclear weapons only since the 1990s. The nation lacks the industrial infrastructure of the other nuclear powers, not only the U.S., but also France, Britain, Russia and even India.

In September, the country detonated a weapon estimated to have a nuclear yield of 15 kilotons to 25 kilotons, according to Siegfried Hecker, former director of the Los Alamos National Laboratory.

North Korea said it had standardized the design and would begin production.

They would be lucky to land 10 miles from their targeting point.— David Wright, Union of Concerned Scientists

Philip E. Coyle III, a retired nuclear weapons design executive for the Energy Department and a former senior Pentagon official, said North Korea’s two-stage Hwasong-14 missile, which flew 45 minutes and reached an altitude of 1,850 miles in space in late July, is significantly smaller than the three-stage U.S. Minuteman III.

Although it appears that the unarmed Hwasong-14 could reach the U.S. mainland, a heavy nuclear warhead would significantly reduce its range.

“I assume what North Korea means by miniature is that it is small enough to be carried by their rocket,” Coyle said. “But this North Korean rocket does not have a lot of payload-carrying capacity.”

A new round of blustery threats emerged over North Korea on Tuesday, as President Trump warned that any military action by Pyongyang “will be met with fire and fury like the world has never seen,” and North Korea’s military signaled it was “carefully examining” a plan to attack Guam.

What could...

(Barbara Demick)

The other key question is whether the North Koreans have an adequate reentry vehicle to carry a nuclear weapon. A warhead travels about 4 miles per second, glowing red hot, as it enters the atmosphere.

The nose cone of the missile has to protect the weapon from the heat and aerodynamic forces of reentry. Its shape has to be nearly perfect to avoid drifting off target.

“The U.S. spent years and years to develop nose cones that would ablate uniformly so the reentry vehicle wouldn’t drift off target,” said David Wright, a weapons expert at the Union of Concerned Scientists. “The missile has to know where it is and where it is going. All the errors add up like crazy,” he added. “My guess is that they would be lucky to land 10 miles from their targeting point.”

CAPTION

Democrat Doug Jones won Alabama’s U.S. Senate seat Tuesday. A cooking fire at a homeless encampment caused the Skirball fire, officials said. San Francisco Mayor Ed Lee, 65, died Tuesday. House Democrats called for an investigation into allegations of sexual misconduct by President Trump.

Democrat Doug Jones won Alabama’s U.S. Senate seat Tuesday. A cooking fire at a homeless encampment caused the Skirball fire, officials said. San Francisco Mayor Ed Lee, 65, died Tuesday. House Democrats called for an investigation into allegations of sexual misconduct by President Trump.

CAPTION

Democrat Doug Jones won Alabama’s U.S. Senate seat Tuesday. A cooking fire at a homeless encampment caused the Skirball fire, officials said. San Francisco Mayor Ed Lee, 65, died Tuesday. House Democrats called for an investigation into allegations of sexual misconduct by President Trump.

Democrat Doug Jones won Alabama’s U.S. Senate seat Tuesday. A cooking fire at a homeless encampment caused the Skirball fire, officials said. San Francisco Mayor Ed Lee, 65, died Tuesday. House Democrats called for an investigation into allegations of sexual misconduct by President Trump.

CAPTION

Democrat Doug Jones, whose uphill bid for U.S. Senate gathered strength when Republican Roy Moore was hit with charges of sexual misconduct with teenage girls, won Alabama’s special election Tuesday.

Democrat Doug Jones, whose uphill bid for U.S. Senate gathered strength when Republican Roy Moore was hit with charges of sexual misconduct with teenage girls, won Alabama’s special election Tuesday.

CAPTION

New York City's bomb attack suspect was allegedly inspired by Islamic State's call for attacks. The Thomas fire has grown to at least 230,000 acres. "The Shape of Water" and "Big Little Lies" led the Golden Globe nominations anounced Monday. Mario Batali stepped away from "The Chew" and his day-to-day business operations.

New York City's bomb attack suspect was allegedly inspired by Islamic State's call for attacks. The Thomas fire has grown to at least 230,000 acres. "The Shape of Water" and "Big Little Lies" led the Golden Globe nominations anounced Monday. Mario Batali stepped away from "The Chew" and his day-to-day business operations.

CAPTION

Santa Barbara County is under siege from the Thomas fire. From the GOP's perspective, Alabama's Senate race will yield one of two unhappy results. The Weingart Center wants to reshape skid row's skyline. If lawmakers can agree on a tax bill, will you owe more or less?

Santa Barbara County is under siege from the Thomas fire. From the GOP's perspective, Alabama's Senate race will yield one of two unhappy results. The Weingart Center wants to reshape skid row's skyline. If lawmakers can agree on a tax bill, will you owe more or less?

CAPTION

An explosive device exploded near Times Square during the morning rush hour Monday, injuring four people and snarling Manhattan transit, authorities said. Mayor Bill de Blasio described it as a terrorist act.

An explosive device exploded near Times Square during the morning rush hour Monday, injuring four people and snarling Manhattan transit, authorities said. Mayor Bill de Blasio described it as a terrorist act.

1:50 p.m.: This article was updated with details about the atomic bombings of Hiroshima and Nagasaki.

This article was first published at 3 a.m.

An earlier version of this article misstated the date of the bombing of Hiroshima, Japan. It was Aug. 6, 1945, not Aug. 9. Also, the "Fat Man" bomb dropped on Nagasaki was referred to as the "Fat Boy." And the first test of a thermonuclear weapon created a blast equal to 10 megatons of TNT, not 19.

An earlier version of this article misstated the date of the bombing of Hiroshima, Japan. It was Aug. 6, 1945, not Aug. 9. Also, the "Fat Man" bomb dropped on Nagasaki was referred to as the "Fat Boy." And the first test of a thermonuclear weapon created a blast equal to 10 megatons of TNT, not 19.